In ink-jet printing methods, droplets of ink are directly projected onto a printing medium from fine nozzles and allowed to adhere to the printing medium to form characters or images thereon and thereby obtain printed materials. The ink-jet printing methods have now been employed not only in printing applications for ordinary consumers but also recently in commercial and industrial printing applications because of various advantages such as easiness of full coloration, low cost, non-contact with printed characters or images, etc.
One of advantages of using the ink-jet printing methods in the commercial and industrial printing applications resides in that since no printing plate is required unlike the conventional analog printing methods, the ink-jet printing methods can be adaptable for production of a small number of prints, i.e., can be used in the applications of on-demand printing such as valuable printing, etc.
In the commercial and industrial printing applications, it has been demanded to obtain printed materials with high productivity as compared to the printing applications for consumers, so that high-speed printing has been carried out in these applications. To cope with the high-speed printing, a print head of a line head printing type has been developed. In the conventional serial head printing method, it has been required to operate the print head (nozzles) plural times. On the other hand, in the line head printing method, only a printing roll paper is scanned while keeping the print head stationary, and it is therefore possible to realize the high-speed printing.
For example, JP 2008-238485A (Patent Literature 1) discloses an ink-jet printing method in which a water-based ink including a pigment and an organic solvent such as dipropylene glycol monomethyl ether is ejected in the form of droplets having a volume of not more than 3 pL from funnel-shaped nozzles arranged in a line print head.
JP 10-53741A (Patent Literature 2) discloses a water-based pigment ink that is used upon conducting sheet printing by a serial head printing method, and includes a pigment, a polymer dispersant, 0.01 to 3% by weight of a specific compound such as polypropylene glycol and a water-soluble solvent.
JP 2011-63630A (Patent Literature 3) discloses an ink composition for ink-jet printing which includes at least a colorant, water, a hardly water-soluble alkanediol and a polyalkylene glycol such as polypropylene glycol, and is used in a serial head printing method.